Why does the concept of exposure value (Ev or EV) cause so much confusion? EV is essentially a very simple way of representing all combinations of shutter speed (exposure time) and aperture which give the same exposure. Consequently, an exposure of 1/250 sec at f/8 has the same EV as one of 1/125 sec at f/11 or 1/500 sec at f/5.6. Each of these pairs of settings allows the same amount of light to reach a camera's film or sensor.However, it may not result in the same image because depth of field changes with aperture and movement may becomes more blurred at lower shutter speeds.
The exposure value scale is, however, also calibrated. It is a base-2 logarithmic scale and therefore corresponds to the conventional range of aperture settings, or stops, marked on typical lenses. An EV value of 0 represents an exposure of 1 sec at f/1.0 and each step on the scale, for example from EV0 to EV1, is equivalent to one stop in terms of exposure. So a change from EV0 to EV1 represents a halving of the exposure. Similarly, a change from EV0 to EV-1 represents a doubling of the exposure.
EV is given by the formula:
EV = Log2 [A2/T]
where EV is the exposure value, A is the aperture value (eg 8, for f/8), and T is the exposure period in seconds (eg 0.04 for 1/25 sec).
EV values, and the corresponding values for aperture and shutter speed, are given in the table below. Values are quoted in seconds except in the upper right-hand corner where the letter "m" is used to denote minutes.
